Scientists who watch for earthquakes have discovered a big increase in the number of small quakes in the middle of the country. It's an area that's usually pretty quiet geologically.

The scientists suspect the quakes are caused by waste-water wells. They plan to discuss their findings later this month at a seismology conference, but they've shared the basics with NPR.

Bill Ellsworth, a seismologist at the U.S. Geological Survey, says new technology over the last decade has given scientists a much better feel for when the Earth shakes.

"We've been watching the seismicity across most of the country very carefully for a number of years now," he says, "and one thing we had begun to notice was there was an unusual number of earthquakes occurring the middle of the country."

Very unusual, in fact. The background rate for the mid-continent had been about 20 quakes a year. That rose to 29 by 2008. Then it really jumped: 50 quakes in 2009, 87 the next year and a whopping 134 last year.

When USGS scientists zeroed in on where they took place, they noticed clusters near waste-water wells, especially in Colorado and Oklahoma. Waste wells are deep holes where various industries pump in waste water at very high pressure. This has been a common practice for decades, and, once in a while, the pumping has created quakes.